1. Field of the Invention
The present invention relates to a mold for producing a glass optical element such as a lens or a prism by press molding of a glass material.
2. Related Background Art
The lens production technology using press molding of glass material without a polishing process has enabled simple and inexpensive production of lenses, without relying on the complex steps required in the conventional technology, and has recently been employed not only in the production of lenses but also prisms and other glass optical elements.
The properties required for the mold employed in such press molding of the glass optical elements include excellent hardness, heat resistance, releasing property and mirror smoothness. For the material constituting such mold, there have been various proposals including metals, ceramics and materials coated therewith. For example, the Japanese Patent Laid-open Application No. 49-51112 proposes 13Cr martensite steel, the Japanese Patent Laid-open Application No. 52-45613 proposes SiC and Si.sub.3 N.sub.4, the Japanese Patent Laid-open Application No. 60-246230 proposes an ultra hard alloy coated with a precious metal, the Japanese Patent Laid-open Application Nos. 61-183134, 61-281030 and 1-301864 propose a thin diamond film or a diamond-state carbon film, and the Japanese Patent Laid-open Application No. 64-83529 proposes a material coated with a hard carbon film.
However, 13Cr martensite steel has the drawbacks of being easily oxidized, and coloration of the glass due to Fe diffusion therein at a high temperature. SiC and Si.sub.3 N.sub.4 are generally regarded as not being oxidized, but are still oxidized at a high temperature, thus forming a SiO.sub.2 film on the surface and causing glass fusion thereto. Also, they have a drawback that the shaping of the mold itself is extremely difficult because of the high hardness. The material coated with the precious metal does not easily cause glass fusion, but is easily damaged or deformed because it is extremely soft.
The mold employing the diamond thin film, the diamond-like carbon (DLC) film, the hydrogenated amorphous carbon (a-C:H) film or the hard carbon film shows satisfactory separation between the mold and the glass and does not cause glass fusion, but may result in an unsatisfactory molding performance due to the partial peeling of such film, after several hundred molding operations. These reasons are considered as follows:
(1) The above-mentioned films have a very large compression stress, and are peeled or cracked as a result of the stress release, caused by the rapid heating and rapid cooling in the molding process. Similar phenomena result from the thermal stress caused by the thermal cycles and the difference in the thermal expansion coefficients of the base material and the film constituting the mold; (2) In certain base materials of the mold, the film may not be formed locally or may be locally thinner depending on the surface state. For example, in a sintered material of WC-Co, SiC or Si.sub.3 N.sub.4, voids of particles or pores in sintering are unavoidable, so that the polished surface contains holes of several microns or larger. If a film is formed on such surface, it may not be formed or may become extremely thin in such holes. Consequently, such locations may induce formation of peeling or crack since the adhesion strength or mechanical strength of the film become extremely low in such locations; (3) The sintering material in the sintered material, as represented by Co in WC-Co, forms an alloy by diffusion with said film. Such portion causes glass fusion in the molding operation, thus generating a precipitate by reaction with the glass components and deteriorating the durability of the mold.
As explained above, the mold for optical elements, excellent in molding property, durability and economical property has not been realized.